1. Field of the Invention
The present invention relates to an induction motor drive apparatus, and more particularly to an induction motor drive apparatus that allows smooth rotation of an induction motor in a low torque and low speed region, such as for mounting a tool on the spindle of a machine.
2. Description of the Prior Art
An operating mode referred to as an orientation mode is adopted when a tool is to be mounted on the spindle of a machine. In this orientation mode, the mounting of the tool on the spindle is performed as follows. Firstly, the spindle is rotated slowly until the respective cutting portions which are formed on both the tool and on the hand portion of the spindle are aligned face to face. At the time when one cutting portion faces the other, the rotation of the spindle is stopped. Then the tool is fixedly mounted on the spindle. Herein, it is considered that one cutting portion faces the other when a pin that projects from the spindle in the orientation mode strikes a stop.
In the above orientation mode, the motor has to rotate smoothly with a suitable driving torque because the spindle and other equipment may be damaged if the driving torque is too large, and not only is the secure operation of mounting of the tool on the spindle not assured, but also the efficiency of machining is lowered, if the driving torque is too small.
So in the orientation mode it is necessary to generate the driving torque whose magnitude is in the range of one third to one half of the driving torque in the cutting mode.
Now, as the motor for driving the spindle, it is possible to utilize DC motors and AC induction motors. From a standpoint of ease of control, a DC motor is efficient because a low speed and low torque control. However, a DC motor has many disadvantages, such as the difficulty of the maintenance of the commutators and brushes, the complexity in construction, the large scale size and high cost. Accordingly, if an induction motor can be utilized instead of a DC motor as the spindle motor, the disadvantages due to the DC motor are avoided. In this case, the following requirements must be satisfied.
(a) In the orientation mode, that is to say in the low torque region the induction motor must rotate smoothly, and
(b) The induction motor must rotate with a suitable driving torque so as not to damage the spindle etc., when the pin strikes the stop.